Plants contain nitrogen and although you can supply it in a fertiliser,
in
the wild it must be absorbed from the air (I presume), but what happens
when
you burn it?

It's absorbed from the soil in the form of nitrates, nitrites and salts
of ammonia, mainly.

You have got to be right Jaques. As soon as I wrote the above, I realised
that if plants could utilise nitrogen from the atmosphere you would never
have to give them a nitrogen fertiliser